Nothing beats snuggling into a sleeping bag at
the end of a long day. But all too often this simple pleasure is
spoiled either because the bag is damp or because it has
insufficient insulation to keep you warm.

Whilst I’ve enjoyed
luxurious nights in sub-zero conditions ensconced in sleeping bags
capable of fending off Antarctic-style temperatures, I have also
been frozen to the bone inside rather more pathetic (and
occasionally soggy) sleeping bags. In the process, I’ve discovered
that whilst there is an art to ensuring the perfect night’s sleep,
it is all too easy to end up being forced to endure a rotten one. I
hope that these notes will allow you to enjoy more of the former and
less of the latter.

Cotswold’s staff have used lots of
sleeping bags and are in the best position to give you advice on
individual products. If you’re after an expert opinion, please chat
to them. Sweet dreams!

Paul Deegan

Paul is the author of the award-winning ‘The Mountain
Traveller’s Handbook’, published by the British Mountaineering
Council and available from Cotswold.

understanding insulation

temperature ratings

Sleeping bags require insulation to trap the heat
that our bodies give off, and to prevent consequent loss of heat
through convection. Without insulation, all of our precious body
heat would escape into the atmosphere, leaving us vulnerable to a
drop in the ambient temperature.

The main principle to bear
in mind with all sleeping bags is that regardless of what materials
they are made with, all are singularly cold and useless until heated
up by a person. Therefore it is critical that you are feeling warm
when you get into your sleeping bag: if you wait until you are
chilled to the bone before slipping into your bag then you’ll be in
for an unpleasant surprise. Even if you are feeling toasty, a
sleeping bag is likely to feel cold immediately upon entering. But
thrash around a little inside and the bag will soon warm up. There
are two clear choices when it comes to sleeping bag insulation:
natural and synthetic. Which type of insulation is most appropriate
for your needs will depend on a number of factors including the
weight you are prepared to carry, the amount of money you are able
to invest, and in what conditions you plan to use the
bag.

Deciding how warm a sleeping bag is makes for a
tricky business. After all, everyone feels the cold differently, and
even the individual will fluctuate in his or her resistance to the
cold from day to day, depending on energy levels as well as
psychological factors.

As I have already mentioned, sleeping
bags are not inherently warm, so it is important to get into your
bag whilst you are giving off plenty of heat. This is best done by
consuming hot food or liquids whilst you are actually in the bag.
Bear in mind that drinking alcohol will force precious core heat to
rush to the skin. Lowering your core temperature in this way is
unhelpful in low temperatures and can lead to
hypothermia.

In 2005 an advisory European standard for sleeping
bags - EN 13537 - was introduced to help ease the comparison of models
from different manufacturers and take some of these considerations into
account. The resulting temperature rating is now a useful guide to
deciding which bag will give you a comfortable night in the conditions
in which it will be used.

The temperature information and
labelling has information that will show four test results – upper
limit, comfort, comfort lower limit and extreme.

‘Upper limit’ - This is the highest temperature at which an
average male user should experience a comfortable night’s sleep.

‘Comfort’ - This is the temperature at which an average female
user should experience a comfortable night’s sleep. On average women
sleep colder than men so this rating is some degrees above the ‘comfort
lower limit’ for a man.

‘Comfort lower limit’ - This is the lowest temperature at which
an average male user should experience a comfortable night’s sleep
whilst lying in a curled up body position. Under the previous tests
this would have been know as the ‘comfort temperature’.

‘Extreme’ - This is a survival rating where the user is likely
to suffer health damage such as Hypothermia. It should be treated with
the utmost caution and not be relied on for general use.

A few points to note:

It is generally accepted that under previous tests the ‘comfort
lower limit’ was based on a fit male user with enough experience to
get the best performance from a sleeping bag. To build in an extra
margin of safety, the new test is based on a user with an average level
of fitness and experience and this is generally reflected in lower
value results.

Temperature ratings are a guideline only. Your age, sex, weight and
fitness all play a part in how you experience cold and you will need
to consider these when making your choice. Be realistic and if in doubt,
consider a warmer model.

Always use an appropriate sleeping mat to get the best performance
from your bag.

Cotswold quotes the ‘comfort lower limit’ rating which is a realistic
starting point for your selection process.

natural
insulation

Natural insulation comes in the form of down
kernels and feathers. Down is a superb insulator; these long-lasting
balls of fluff are the warmest and most compressible filling that
money can buy. Feathers, by contrast, trap very little heat. A
decent down bag will possess at least 85% down; the very best bags
boast 95% or more.

But there’s more to down than mere
percentages. The other key consideration is the strength of the
down, which is measured by placing a specified amount of down in a
container and measuring how much room it occupies under pressure.
The result is known as a fill power rating, and reputable
manufacturers regularly test the down they receive from suppliers in
order to ensure it meets their requirements. Down with a fill power
of 550 is often used in mid-price bags. Anything over 650 is
excellent. A few top-end bags claim a fill power of 750- plus. (Note
that these figures are for down samples subjected to the European
fill power test: some other countries employ a fill power test which
delivers higher results for the same quality of down).

Yet
down is not without its disadvantages.When wet it is useless, and it
takes an age to dry out. Finally, down needs to be cared for very
carefully.

synthetic insulation

keeping your bag
dry

Many people opt for the convenience of a bag
filled with synthetic insulation. And with good reason. Most
synthetic bags are filled with polyester filaments. These are
designed to trap body heat just like down kernels, but without any
of the natural fibre’s disadvantages. A synthetic bag will typically
retain about half of its insulation value when wet, which makes it a
more appropriate choice for damp climates such as the UK and
Patagonia, as well as perennially humid environments like tropical
rain forests. A synthetic bag requires little or no care when it
comes to cleaning or drying out. And although a synthetic bag’s
useful life is approximately half that of down, prices are
correspondingly lower.

However, a bag containing synthetic
fibres needs more filling than a down bag in order to work
efficiently in a similar temperature range. So if low weight is your
first priority then a down bag with a high fill power rating might
be more suitable for your needs.

Another type of synthetic
bag is made from fibre-pile, and is typically covered with a
windproof nylon. Bulky and relatively heavy for a warm(ish) model,
fibre-pile has the advantage of continuing to trap heat very
efficiently even when saturated. Pile dries out quickly too, making
it an excellent choice for
mariners.

As we have seen, most types of sleeping bag
filling lose some or all of their insulating qualities when wet.
However, moisture can enter the bag not only from outside in the
form of rain and snow but also from the inside.

One tip for
keeping your bag dry is to understand the dew point and how it can
make your sleeping bag damp from within. The dew point is the point
at which your sweat vapour condenses into a liquid. Normally this
occurs outside the bag after your sweat vapour – which can amount to
a litre or more per night – has passed through the insulation.
However, if your sleeping bag lacks sufficient insulation to stave
off the ambient temperature, the dew point can creep inside the
filling. If this happens then the bag will start to become damp. In
really low temperatures, your sweat can freeze inside the bag,
forming ice crystals within the insulation. Placing an ultralight,
oversized pile or synthetic bag around a down bag in these
conditions will help to prevent the dew point from moving into the
down.

At any rate, some vapour is likely to be left in the
bag at the end of the night and so regular - preferably daily -
airing is essential in order to keep your bag dry and maintain the
thermal efficiency of the filling.

It is also worth bearing
in mind that you breathe out a lot of vapour through your mouth. If
you choose to sleep with your head buried inside your sleeping bag,
the area around your face will become very damp. I find that it is
better to sleep with my face exposed, even if that means wearing a
face mask or balaclava in cold weather.

Sleeping bags can get
also get wet from rain, snow, tea spills, river crossings and sodden
clothing. The latter two are easily dealt with: when travelling,
pack your sleeping bag in a kayak-style waterproof bag, and avoid
putting damp clothes into your sleeping bag. As for rain and snow,
the first protection is a decent tent. If this is impossible due to
the nature of the undertaking, then a bivi bag can provide an
effective alternative.

When it comes to preventing tea spills
whilst camping, it's definitely worth investing in an inexpensive,
insulated plastic mug and lid. Many manufacturers use
water-resistant nylon materials on the outside of their bags, so
with luck you should be able to simply shake the worst of any spill
from the bag as soon as any accident occurs. A small sponge is handy
for mopping up the excess
moisture.

enjoying a better night’s
sleep

At night you lose more heat through the ground
than through the air, so try to ensure that you always lie on a foam
mat (either closed cell or self-inflating open-cell). Place any
spare clothing between the mat and the sleeping bag. In particularly
cold temperatures fold your mat in half, place it under your torso,
and put your legs on top of your empty rucksack. It’s easier to cool
a warm body than heat a chilled one, so in cold weather utilise all
the features of your bag. For example, tighten the shoulder collar
and hood drawstrings in order to create a heat-trapping seal around
your face and neck.

Getting up in the middle of the night for
a pee is a quick way to get cold. But holding on until morning is
little better as your body will have to divert precious heat to keep
the urine in your bladder warm. So consider taking a clearly
labelled, leakproof pee bottle to bed. (Ladies will find a funnel
useful). In sub-zero conditions, either leave the full bottle in
your bag or reach outside and empty it. Otherwise, you’ll end up
having to defrost a litre of frozen yellow liquid in the morning! If
you are still not enjoying a better night’s sleep, it may be that
your bag simply does not have sufficient insulation to keep you
warm. If your sleeping bag is roomy, buy a thick fleece liner or a
thin one-season sleeping bag and slip it inside. Alternatively, buy
an oversized one-season bag and put your existing bag inside
it.

caring for your sleeping
bag

design &
construction

Sleeping bags come supplied in tight-fitting
stuff sacs. These should only be used during a trip when a small
packed size is important. Back at home and it is much better to
store your bag in a large cotton sack, hang it in a cupboard or lie
it flat under a bed. Keeping the bag uncompressed in this way will
maximise its useful life.

A dirty sleeping bag works less
efficiently than a clean one. Using a liner will help to keep the
bag clean. Cotton liners are cheap and cheerful, but soak up sweat
and can take a while to dry. Silk liners are more expensive, much
lighter and dry more quickly.

When your sleeping bag becomes
a health hazard, follow the cleaning instructions closely. Washing a
sleeping bag takes time and effort, so choose a warm day well in
advance of your next trip. Down bags are particularly troublesome to
wash, and so many owners prefer to take advantage of the services of
a specialist such as W.E. Franklin (tel: 0114 268 6161), who
professionally wet clean down bags – and duvet jackets – for a
reasonable price. On no account have your bag dry-cleaned; the
chemicals used in the process can damage the bag and the fumes given
off are toxic. Companies such as Mountaineering Designs (tel: 015395
36333) will re-cover old down bags in order to give them a new lease
of life. Extra grams of new down can be stuffed in at the same time
if required.

One of the first things to consider when choosing
a sleeping bag is where you are going to use it. If the bag is
mainly going to be used inside hostels, then a rectangular design
with a L-shaped zip (which allows the bag to be thrown open
quilt-style) will probably suffice. At the other extreme, a
mountaineer wanting the most heat-conserving type of sleeping bag
will almost always opt for a snug-fitting sarcophagus design. This
type of ‘mummy’ bag reduces the amount of dead air in the sleeping
bag, and so provides maximum warmth albeit at the expense of some
comfort.

Backpackers and trekkers often opt for a tapered bag
which offers some of the heatretaining benefits of the mummy shape
but also allows room for manoeuvre inside the bag. Bags destined for
service in colder climes will have features such as shoulder
baffles, zip baffles, sculptured hoods, and intricately designed
foot sections which help to keep toes warm. The main features of a
typical winter sleeping bag are illustrated below:

We’ll discuss the different types of sleeping bag
insulation in a moment. But whatever filling you decide upon, bear
in mind that it needs to be held in place over and around the body.
Synthetic insulation is usually supplied in batts and so is easy to
sew up in a single layer (for summer use) or in a double, offset
layer (for year-round use). Down, on the other hand, is a
free-moving product which must be blown into cubes of material. This
process is timeconsuming and expensive when compared to the
construction of synthetic bags.